One known type of water heater provides a water tank and a burner located below the tank arranged so that combustion products from the burner are directed to flow up the sides of the tank to thereby heat the water in the tank. The tank is mounted and centred within a casing so as to define a flue path for the combustion products between the outside surface of the tank and the casing. An example of this kind of water heater is shown in the present assiginee's Australian patent specification No. AU-28685/97 (Patent No. 720,062).
A problem or difficulty with this kind of arrangement is that the heat extraction efficiency may not be optimised and, in particular, combustion products may still have some significant heating capacity after being wastefully discharged from the unit.
Another kind of water heater, such as shown in Australian patent specification AU-38573/93, uses a flue for combustion products passing vertically through the centre of the water tank. The flue is provided with convolutions and baffles intended to improve the heat extraction efficiency. It would be possible to use convoluted shapes and baffles along the outside of the tank of the kind shown in our patent specification AU-28685/97 but this would be complicated and undoubtedly very costly to manufacture and assemble.